Gold plating is often applied to electronic parts and many other articles. Most gold plating is partial plating of selected areas of an article following masking of the remaining areas where no plating is necessary. Such partial plating requires a high precision of masking particularly in plating of electronic parts. A variety of masking agents and methods have been developed to meet such requirements. Since masking agents are generally less resistant to alkali, plating in an alkaline bath sometimes causes the masking film to separate from the underlying substrate. On the other hand, electronic parts now use a variety of substrates, some of which are less resistant to alkali. There is a need for an acidic bath for partial plating purposes.
Most prior art gold plating baths use gold compounds in the form of cyanides. Although cyanides, halides, sulfites, and thiosulfates are known as water soluble gold compounds, the gold source for gold plating bath is most often gold cyanides because of their shelf stability. The gold cyanides include potassium aurous cyanide or gold(I) cyanide and potassium auric cyanide or gold(III) cyanide. Potassium aurous cyanide is most often used in the current gold plating bath.
The plating bath using potassium aurous cyanide becomes ineffective at an acidity of pH 3 or lower because potassium aurous cyanide decomposes into AuCN, that is, becomes insoluble in water. Thus the potassium aurous cyanide plating bath is formulated as weakly acidic type with at least pH 4, neutral type and alkaline type, but not applicable to strongly acidic bath of pH 3 or lower.
In contrast, potassium auric cyanide is stable at an acidity of pH 3 or lower and used in some plating baths. However, such plating baths are not widespread because of their preparation cost.